The 2-point form of the equation for a line is applicable. For points (x₁, y₁) and (x₂, y₂) that equation is ...
... y = (y₂ -y₁)/(x₂ -x₁)·(x -x₁) +y₁
Filling in your point values gives ...
... y = (3 -1)/(0 -(-3))·(x -(-3)) +1
This can be simplified to ...
... y = (2/3)(x +3) +1
... y = (2/3)x +3